The invention concerns a method in the manufacture of paper or board for dewatering of a paper web that is being manufactured and that has been drained in the former of the paper machine. In the method, the dewatering takes place by passing the paper web on support of fabrics that receive water through a number of subsequent dewatering nips. In this manner, by the effect of the compression pressure, water is transferred out of the fiber mesh of the paper web into the spaces in the fabric that received water as well as into the spaces in the hollow faces of the mobile dewatering members, such as press rolls.
Further, the invention concerns a press section of a paper machine, into which the paper web to be dewatered by pressing is passed from the former of the paper machine and from which the paper web is passed into the drying section of the paper machine. The press section comprises at least two separate press-nip zones, two press fabrics that receive water passing through at least the first one of said press-nip zones, between which fabrics the web runs through said nip zone.
One of the most important quality requirements of all paper and board qualities is uniformity of the structure both microscopically and macroscopically. The structure of paper, in particular of printing paper, must also be symmetric. The printing properties required from printing paper include good smoothness, evenness and certain absorption properties at both faces. The properties of paper, in particular the symmetry of density, are affected considerable by the operation of the press section in a paper machine, which has also a decisive significance for the evenness of the transverse profiles and longitudinal profiles of the paper.
Increased running speeds of paper machines provide new problems, which are mostly related to the running quality of the machine. At present, running speeds of up to about 1400 m/min are employed. At these speeds, so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced center roll, usually operate satisfactorily. Such press sections are commercially available from the assignee of the present application, Valmet Oy, under the tradenames "Sym-Press II.TM." and "Sym-Press O.TM.". One item that requires development is the center roll in the compact press sections and the material of the roll, which has commonly been rock, which, however, being a natural material, has certain drawbacks.
Dewatering by means of pressing is energy-economically preferable to dewatering by evaporation. This is why attempts should be made to remove a maximum proportion of water out of a paper web by pressing in order that the proportion of water that must be removed by evaporation could be made as low as possible. However, the increased running speeds of paper machines provide new, as yet unsolved, problems expressly in the dewatering taking place by pressing, because the press impulse cannot be increased sufficiently by the means known in prior art. This is because, at high speeds, the nip times remain unduly short and, on the other hand, the peak pressure of compression cannot be increased beyond a certain limit without destruction of the structure of the web.
When the running speeds of paper machines are increased, problems of running quality of paper machines are also manifested with increased emphasis, because a watery web of low strength cannot withstand an excessively high and sudden impulse of compression pressure or the dynamic forces produced by high speeds, but web breaks and other disturbance in operation are produced with resulting standstills. With a modern printing paper machine, the cost of a break standstill is at present about 40,000 FIM, about $8,000, per hour.
Further drawbacks of the prior art press sections include the requirement of suction energy of the suction rolls commonly employed in them as well as the noise problems arising from the suction rolls. Also, the suction rolls with their perforated mantles, interior suction boxes, and other suction systems are components that are expensive and require repeated servicing.
Further problems which are manifested with more emphasis at high speeds of paper machines and for which, at least not for all of them, satisfactory solutions have not yet been found, include the quality problems related to the requirements of evenness of the longitudinal and transverse property profiles of the paper web. The evenness of the web that is produced also affects the running quality of the whole a paper machine, and it is also an important quality factor or finished paper, which is emphasized in respect of copying and printing papers when the requirements on the speeds of copying and printing machines and on the uniformity of the printing result are increased. The property profiles of the paper that is produced in the machine direction are also significantly affected by oscillations of the press section, the transverse variations of properties by the transverse profiles of the nip pressures in the press nips. With increasing running speeds of the machine, these profile problems tend to be increased remarkably.
Recently, running speeds even as high as about 40 m/s, or 2400 m/min, have been contemplated as running speeds of paper machines. The realization of such high speeds, in particular in wide machines, creates ever more serious problems to be solved, of which problems some of the most important ones are the running quality of the machine and adequate dewatering capacity at high speeds.
With respect to the prior art most closely related to the invention, reference is made to U.S. Pat. Nos. 4,483,745 (Beloit Corp.), 4,526,655 (Valmet Oy), 4,561,939 (Beloit Corp.) as well as to the published patent applications WO-85/00841 (J. M. Voith GmbH), DE-OS-3742848 (Sulzer-Escher Wyss GmbH), and to the FI Patent Applications 842114 (Valmet Oy), 842115 (Valmet Oy) and 850665 (Valmet Oy).